In acetic acid fermentation, since alcohol is an inhibitory substance as well as a substrate for acetic acid production, its concentration in a fermenting broth remarkably influences the fermentation productivity. If the alcohol concentration is far away from an optimal concentration, the fermentation rate not only at the present time but also at a subsequent time period of fermentation, would drop and result in a considerable decrease of the productivity of acetic acid.
However, a method for controlling the feed rate of alcohol solution as raw material on the basis of an experienced prediction of a future stage of fermentation by taking a part of a fermenting broth and measuring subsequently the alcohol concentration by means of conventional methods, such as a gas chromatography and colorimetric method, has been customarily used to carry out a monitoring. In this conventional method, it takes too much time for a series of procedures consisting of a collection of a fermenting broth for alcohol measurement and an adjustment of the feed rate after the alcohol measurement, and also the frequency of measurement is restrictive. Therefore, it is impossible to accurately follow or track the fermentation stage without excessive delays. Since the fermentation proceeds moment-by-moment, day-and-night, this conventional method is generally no longer used to always maintain the alcohol concentration in a fermenting broth optimal during fermentation.
Recently, a method which monitors the fermentation by interfacing various sensors and using a computer has become well-known. For example, in Baker's yeast fermentation, the feed rate is controlled by computer using a calculation based on the analytical result of carbon dioxide and oxygen concentration in exhaust gas. In acetic acid fermentation, it has been well-known that an important parameter is alcohol concentration in a fermenting broth as described above. However, since no equipment capable of measuring accurately and rapidly the alcohol concentration in an acetic acid fermenting broth has been developed so far, and also since a method of controlling accurately the alcohol concentration in a fermenting broth by controlling the feed rate of alcohol solution as raw material without any effect by more or less time delay has been unknown, there is no example so far of any system wherein the feed rate of alcohol solution as raw material was controlled using a computer in acetic acid fermentation.
A well-known control method which controls the feed rate on the basis of oxygen uptake with a sealed-fermentor is disclosed in Japanese Tokkyo Kokai Koho No. 57-181685. A known method which controls the feed rate of an alcohol solution on the basis of measurement of alcohol concentration using the temperature difference of boiling points gained by heating a fermenting broth taken out from a fermentor is disclosed in Japanese Tokkyo Koho No. 47-22799. However, these known methods were not satisfactory for the demand of vinegar industries which has wanted to control alcohol concentration in a fermenting broth accurately and always optimal, because these known methods have disadvantages such as that they were not useful in an opened fermentor which is, in general, used in the vinegar industry, and that they have a time delay of more than 30 minutes and, therefore, have poor accuracy of measurement in high alcohol concentrations.